How to treat fungus gnats in your houseplants
Published on November 4, 2024 at 11:17am CST
Growing Green
By Robin Trott, Extension Educator
As cooler weather brings houseplants indoors, many plant owners are dealing with fungus gnats. These tiny, mosquito-like insects hover around potting soil and, while harmless to humans, they can stress plants—especially seedlings and delicate roots. Here’s how to identify and treat fungus gnats and keep them from returning.
What Are Fungus Gnats?
Fungus gnats are small, dark flies that lay their eggs in moist soil. Their larvae feed on decaying organic matter and sometimes plant roots, which can cause yellowing leaves, wilting, and slow growth. Fungus gnats thrive in damp conditions, so overwatered plants are especially vulnerable.
How to Spot Fungus Gnats
If you see tiny black flies around your plants or windows, you may have fungus gnats. To check, tap the pot lightly; gnats will take flight if present. You may also see tiny, white worm-like larvae on the soil surface.
How to Treat Fungus Gnats
1. Dry Out the Soil
Fungus gnats love moisture, so let the
top 1–2 inches of soil dry completely
before watering again. This disrupts
their life cycle, as larvae can’t survive
in dry soil.
2. Use Sticky Traps
Yellow sticky traps, available at
garden centers or online, easily catch
adult fungus gnats. Place them near
the base of plants and replace as needed.
This reduces the number of adult gnats
laying eggs.
3. Top the Soil with a Barrier
Cover the soil with a thin layer of sand,
diatomaceous earth, or fine gravel.
This barrier makes it harder for adult
gnats to reach the soil and lay eggs.
A layer at least half an inch thick is best.
4. Water from the
Bottom
Bottom-watering
lets the plant absorb
only what it needs,
keeping the soil’s sur
face dry and discour-
aging gnats. Set the pot in a tray of water
for 15–30 minutes, then remove
any excess water.
5. Use Biological Insecticides
A natural bacterium called Bacillus
thuringiensis var. israelensis (BTI),
found in mosquito dunks, works
against fungus gnat larvae. Add a piece
of a dunk to your watering can. BTI is
safe for plants and pets and targets
only fungus gnat larvae.
Preventing Fungus Gnats
After treatment, use these steps to keep fungus gnats from returning:
• Avoid Overwatering: Stick to a watering schedule and use pots with drainage holes.
• Remove Dead Leaves and Debris: Clean up dead plant material, as decaying matter attracts gnats.
• Use Well-Draining Soil: A well-draining mix helps prevent overly moist conditions that fungus gnats love.
Fungus gnats can be annoying, but a few simple changes to your plant care routine can get rid of them. Drying out soil, using sticky traps, and treating with BTI are all effective ways to protect your plants and keep your indoor garden pest-free.
With these tips, you can enjoy your houseplants without the constant buzz of unwelcome pests. Happy gardening!
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